1. Field of the Invention
The subject invention generally pertains to a system for detecting the presence of a body near a doorway and more specifically to a system that helps prevent a door from accidentally closing against the body.
2. Description of Related Art
There are a wide variety of available devices for detecting the presence of a body, such as a person or object, near a doorway. Such detection devices, known as photoelectric eyes, proximity sensors, motion detectors, operate under various principles including, ultrasonics; active and passive detection of infrared radiation; detection of electromagnetic radiation (including sensing radio waves or sensing changes in capacitance or inductance); and detecting a Doppler shift in microwaves; and lasers. In response to sensing a nearby body, the detector may simply trigger a light or an alarm, or the device may affect the operation of a door.
In door applications, a detection device generally falls under one of two categories: a door opener or a door interrupter. A door opener triggers the opening of a door for an approaching body, such as a shopper entering or leaving a store. A door interrupter, on the other hand, prevents an already open door from accidentally closing against a body that may be in the doorway or within the path of the door's travel.
Door openers typically monitor an area in front of the door where the approaching body is expected to travel. Since door openers are more for convenience than safety, the monitored area is a general vicinity rather than a tightly controlled, well defined area in front of the door. Often, the monitored area does not extend the full width of the doorway. So, in many cases, a body may avoid detection by approaching the door from the side, thereby reaching the door without the door being automatically opened. Such operation may be acceptable for a door opener, but a door interrupter preferably provides more complete coverage to minimize the possibility of an approaching body avoiding detection.
Some door interrupters comprise an antenna that creates an electromagnetic field along the leading edge of a vertically operating door. When a nearby body disturbs the field by coming within a few inches of it, the door interrupter may respond by stopping or reversing the closing action of the door. Since the antenna, and thus its field, moves up and down with the leading edge of the door, somebody may be tempted to “beat the door” by racing underneath a closing door before the interrupter can sense their presence.
Some reliable door interrupters have a horizontal activation line that is about 24-inches above the floor and extends completely across the width of the doorway. So, anything taller than the height of the activation line would have to trigger the door interrupter upon passing through the doorway. Since activation lines of such door interrupters typically lie immediately adjacent to the door, an approaching body typically will not trigger the interrupter unless the body is within or right next to the doorway.
Consequently, there is a need for a door interrupter whose field of view is broader than current door interrupters and more complete and well defined than current door openers.